Optical sensors, particularly fiber-optic sensors, are very attractive for the measurement of temperature in applications where immunity to electromagnetic interference, intrinsic safety and small size are required. Various optical methods have been proposed for temperature sensing. These methods can be classified by the specific properties of the light radiation (intensity, phase, polarization, and spectrum) to be modulated by the temperature. Among them, optical interferometry, which concerns the phase modulation of the light radiation, is recognized as one of the most sensitive method for optical temperature sensing by means of the temperature-induced changes of the interferometer path length difference. Although this method looks very attractive for temperature measurement, it may have severe restrictions if not properly applied. It is well known in the art that when using a narrowband light source, the coherence length of the light source is generally greater than the path length difference of the interferometer and therefore the measurement has a modulo 2π phase ambiguity, due to the periodic nature of the interferogram fringes. The modulo 2π phase ambiguity problem is avoided by using a light source with short coherence length. In this case, the fringes of the interferogram are narrowly localized into a path length difference region so the variation of the path length difference can be determined without the modulo 2π phase ambiguity by locating the fringe peak or the envelope peak of the interferogram. This type of interferometry is known as white-light or low-coherence interferometry (LCI).
LCI-based temperature optical sensor generally requires two interferometers usually connected with an optical fiber: 1) the sensing interferometer, which is subjected to the environmental temperature to be measured, and 2) the readout interferometer which is used to measure the temperature-induced changes of the path length difference of the sensing interferometer. This configuration is known in the art as the tandem interferometer arrangement.
A number of optical sensors for measuring temperature have been already proposed. U.S. Pat. No. 4,140,393 Cetas, February 1979 and U.S. Pat. No. 4,598,996 Taniuchi, July 1986 disclose the use of different birefringent crystals in a two-beam interferometer configuration as the sensing element for measuring temperature. They use crystals such as LiTaO3, LiNbO3, BaTiO3 and SrxBa1-xNb2O6 (SBN) to form a polarization interferometer and they measure the light intensity at the output of this sensing interferometer which varies sinusoidally due to temperature-induced changes of the crystal birefringence. Their optical sensing system is based on narrow-band light source so their measuring technique suffers from a modulo 2π phase ambiguity and therefore offers a limited measurement range.
U.S. Pat. No. 5,255,068 Emo et al., October 1993 uses crystals and sensing interferometer arrangement similar to those of Cetas and Tanaiuchi for measuring temperature but their optical sensing system benefits from the short coherence length of the light source they use. However, the light source spectrum, modulated according to the temperature-dependent birefringence of the crystal, is recorded using a dispersive spectrometer which is known to have a low optical throughput. Since the resulted signal is obtained in the frequency or wavelength domain rather than in the time or spatial domain, they use a Discrete Fourier Transform signal processing method which can be time consuming without mentioning the cost and complexity of using a dispersive spectrometer configuration. Moreover, the above-mentioned crystals are known to have a strong frequency-dependence of their birefringence (birefringence dispersion) which can severely compromise their measurement method.
U.S. Pat. No. 5,392,117 Belleville et al., February, 1995 and the document by Duplain et al. “Absolute Fiber-Optic Linear Position and Displacement Sensor” published in OSA Technical Digest Series, Vol. 16, 1997 describe the use of a Fabry-Perot interferometer as a sensing interferometer for measuring various physical quantities including temperature and a readout interferometer, namely a Fizeau interferometer made of an optical wedge, to measure the measurand-induced changes of the path length difference of the sensing interferometer. Their LCI-based optical sensing system consists of recording the fringes of the interferogram at the output of a Fizeau readout interferometer using a linear photodetector array and to locate the fringe peak position on the interferogram. One advantage of using a Fizeau interferometer is related to its static nature, that is, with such a type of interferometer the interferogram is recorded in space rather than in time (as for dynamic interferometers) so none of the interferometer optical components are intended to move during a measurement. The Fizeau interferometer disclosed by Belleville et at and by Duplain et al may have material dispersion which can be detrimental to the localization of the fringe peak of the interferogram.
FISO technologies Inc. commercializes a Fabry-Perot sensing interferometer (FOT models) which uses a temperature transduction mechanism based on the thermal dilatation of one or both of optical glass fibers that form the mirror supports of the interferometer. Consequently, the temperature-induced changes of the path length difference rely on the mechanical properties rather than on the optical properties of the optical glass fibers. For those skilled in the art, it is known that amorphous glasses can suffer from hysteresis in thermal dilatation due to the inherent thermal expansion mismatch between the different materials that compose the interferometer. Thermal-creep is also a well known problem encountered with amorphous glasses and this may affect the long term accuracy of this type of sensor.
U.S. Pat. No. 4,814,604 and U.S. Pat. No. 4,867,565 issued to Lequime, as well as the document by Mariller and Lequime entitled “Fiber-Optic White-Light birefringent temperature sensor” published in SPIE Proceedings, Vol. 798, 1987, disclose the use of a LCI-based optical sensing device including a sensing interferometer for temperature measurement similar to the configuration disclosed in Cetas and Taniuchi patents. Their LCI-based optical sensing system consists of recording the fringe pattern at the output of a readout interferometer using a linear photodetector array (static interferometer configuration) or a single photodetector (dynamic interferometer configuration). Their polarization-based readout interferometer is a rather complex assembly of different birefringent elements placed in between two polarizers. The birefringent elements comprise, at least, a crystal plate with two elementary birefringent prisms stuck together along a face so to form a Wollaston or a modified-Wollaston prism. These birefringent elements are mounted in variant forms of the Babinet compensator and the Soleil compensator. These types of configurations produce complex assembly devices and suffer from important drawbacks. In it simplest configuration, the plane of localization of the fringes is inside the Wollaston prism and is inclined to the exit face of the Wollaston prism. This situation requires correction optics to form an image of the fringes onto the surface of the photodetector. However, the inclination of the plane of localization produces a residual focusing error at the surface of the photodetector and therefore leads to a reduction in the fringe contrast unless the light source has a high degree of spatial coherence. To prevent this situation, Lequime proposes some modifications in their initial configuration by using a second Wollaston prism and an achromatic halfwave plate, but at the expense of increasing the complexity of the device.
Due to the high birefringence dispersion of the crystal used in their sensing interferometer (and possibly in the readout interferometer) the interferogram can be severely distorted therefore compromising the localization of the envelope peak or the fringe peak. Mariller and Lequime propose two solutions to overcome this problem. One solution consists of using a readout interferometer made of same birefringent material to that of the sensing interferometer. Such solution is likely to increase the sensitivity of the readout interferometer to environmental temperature influences and therefore is not desired for industrial-based applications. A second solution proposed is the use of a light source with a narrower spectrum resulting into a reduction of the dispersion effects. This solution comes to the expense of widening the path length difference region of the fringes which inevitably reduces the accuracy of the envelope peak or the fringe peak location.